1. Field of the Invention
The present invention is directed to communications-related systems, networks apparatuses and methods as well as computer-based digital signal processing mechanisms and methods used therein. More particularly, the invention is directed to the field of direct sequence spread spectrum (DSSS) communication that employ a DSSS transmitter and a DSSS receiver, or transceiver configured to convey a data signal in a transmitted DSSS signal by spreading the data signal on transmission and correlating on reception so as to "despread" the DSSS signal and recover the data signal.
2. Discussion of the Background
Conventional narrowband (i.e., non-spread spectrum) radio communication devices transmit signals in frequency bandwidths that are roughly equivalent to a data signal bandwidth (or information bandwidth). These devices typically use a radio-frequency (RF) carrier derived from a frequency reference (i.e., a device that produce a precise frequency, although the accuracy of the frequency usually depends on the cost of the device) and modulate the data on the RF carrier. Common conventional data modulation methods such as frequency modulation (FM), phase modulation or amplitude modulation (AM) cause the RF carrier to occupy slightly larger bandwidth then the RF carrier alone, but the total bandwidth for the RF carrier and data is not much larger. As such, interference signals (e.g. jammers) that are transmitted in the same bandwidth as the RF carrier and data can effectively "jam" the signal and prevent a receiver from reproducing the data signal. Aside from jamming, disturbances in the communications path between the transmitter and receiver can interfere with reception. For example, fading due to multipath or atmospheric obstruction can attenuate the signal significantly. Also, shadowing becomes significant if the signal must pass through solid matter such as buildings, walls, floors or trees and vegetation.
Spread spectrum radio communication addresses the shortcomings of narrowband communications by mixing (i.e. applying) a wideband spreading signal to the data signal so as the "spread" the data signal. In these types of systems, the transmitter also modulates a RF carrier with data, as with the narrowband systems, but then adds one more modulation step by modulating the signal with a wideband, noise-like signal (e.g. a PN code). Consequently, the data signal is spread in frequency over a much larger bandwidth, typically several million Hertz (MHZ). Common spread spectrum techniques include frequency hopping and DSSS. Frequency hopping systems move (i.e., "hop") the data modulated carrier to frequencies following a pseudo-random pattern defined by the PN code. DSSS mix a PN code with the data modulated carrier to create a DSSS signal which simultaneously occupies roughly the bandwidth of the pseudo noise signal.
Narrowband interference signals transmitted at same frequency as a portion of the spread signal, "jam" the spread signal by an amount proportional to the ratio of jammer bandwidth to pseudo-noise bandwidth. At a minimum, the interference signal will at least be attenuated by a "processing gain" of the spread spectrum signal, where processing gain is defined as a ratio of data signal bandwidth to spread signal bandwidth. For similar reasons, spread spectrum signals also offer some degree of immunity to channel fading and multipath loss.
DSSS systems have been used in the past to achieve low probability of intercept (LPI) for secure communication and thus are valuable in military applications or other scenarios requiring covert communications. DSSS is also used in places where multipath or fading is prevalent, such as satellite communication. For example, Global Positioning System (GPS) operates using DSSS techniques. However, as recognized by the present inventors, conventional DSSS systems are expensive (considering the transmitter and receiver) because relatively high performance frequency references and digital signal processing equipment is used. Accordingly, DSSS techniques are most commonly used in military and high-end consumer market, where component cost is less of a factor than with low-end consumer product.
Conventional direct sequence spread spectrum transceivers are directed towards high-end systems (e.g. systems costing in the hundreds or thousands of dollars in 1997) that require advanced, if not state-of-the-art, digital signal processing equipment, and associated components. Past DSSS systems have avoided using lower cost components because conventional wisdom dictates that selectively high fidelity frequency references are required at the transmitter and receiver, as well as powerful digital signal processing equipment so to compensate for even minor frequency deviations between transmitter and receiver systems. Contrary to conventional DSSS design practice, the present inventors have identified that these conventional DSSS devices are not applicable for low-end, inexpensive, commercial use applicable for high-volume sale, nor are they well suited for small packages, that may be used in a variety of non-standard field uses, such as, for example, home security and fire systems, data telemetry, access control, remote meter reading as well as other applications.
As recognized by the present inventors, one factor that drives the cost of conventional systems is the use of lengthy PN codes that require substantial digital signal processing to be despread in a receiver. While there are many advantages to using a long code (such as with code division multiple access, CDMA, telephony which permits many users to transmit on a common channel at the same time) the present inventors have recognized that a shorter code, such as a 63 bit PN code, may enable the use of components applicable for lower cost applications.
FIG. 1 is a block diagram of a conventional receive system that of either a conventional receiver (either narrowband or DSSS receiver). The receive system includes a RF front end section 120, a first local oscillator (109, 112, 111, and 110, as will be discussed) section, an analog-to-digital conversion section 121, baseband mixing section (115, as will be discussed) and a baseband processing section 122, as shown. The details of the conventional receiver are described below, following a general overview description. The RF section 120 performs the function of converting electromagnetic wave energy (including the transmitted signal) and outputting an analog signal. The analog signal is maintained within a predetermined signal level range, as controlled by an automatic gain control circuit (AGC) as shown. The output from the RF front end is provided to a first local oscillator section having a mixer 109, which translates the analog signal to a lower frequency by using a precise, and generally expensive, voltage controlled oscillator 110. By employing the precise voltage controlled oscillator 110, the position of the translated signal (i.e., a down converted signal), is controlled to within a narrow predetermined frequency range.
The downconverted signal is then passed to the intermediate frequency processing section 121, that adds appropriate gain prior to a digitalization process while filtering out-of-band images necessary for the digitization process, as will be discussed herein. The output of the intermediate frequency section is passed to the analog to digital converter ADC shown as mixer 115 (as will be discussed), which converts the analog signal into a digital representation for subsequent processing in the baseband section 122.
In this conventional architecture, the analog AGC's function is to keep the signal level applied to the ADC 115 within an operational range of the ADC. Once digitized, the signal is passed to the baseband section 122, where digital signal processing operations are performed on the signal and the signal is detected and demodulated, resulting in outputting the data signal originally transmitted from the transmitter (either in a spread or non-spread form).
In the special case of direct sequence spread spectrum receivers, the burden of performing the "inverse spreading" (despreading) operation on the signal usually falls on the digital signal processor 116 section 1 of the baseband processing section 122. In some cases, the despreading code is mixed in the RF front end 120, first LO or intermediate frequency (IF) sections of the receiver, but such analog architectures require significantly precise components or specialized compensation mechanisms. Accordingly, conventional direct sequence spread spectrum receivers, require high performance digital signal processors or complicated analog sections in order to perform the despreading and correlation functions, as well as signal acquisition and demodulation processes. As a corollary, a precise frequency reference at the DSSS transmitter is assumed to be present so little to no frequency ambiguity is presented into the signal received by the receiver.
Conventional digital receiver design wisdom is such that the loss in performance associated with using low cost, low power desirable and possible components does not justity their use in light of the fact that slightly more expensive components provide greater precision and processing power and therefore avoid performance problems associated with low-cost, inaccurate components. As identified by the present inventors, the high performance digital signal processors, as well as high fidelity voltage controlled oscillators, are not conducive to low cost, low power applications, and that with proper compensation mechanisms, the adverse effects of low cost components can be justified.
More particularly, FIG. 1 depicts RF downconversion and signal conditioning components used to prepare a received analog signal for sampling. Antenna 101 receives the RF signal sent from a transmitter and passes the signal through RF diversity switch 102 and on to a bandpass filter (BPF) 103. Alternatively, a controllable select mechanism 104 may select antenna 105 to receive the RF signal. The antennas may be physically separated and/or of different polarizations so as to enable spatial or polarization diversity reception. The BPF 103 rejects undesired frequencies prior to signal amplification by an amplifier 106. The RF signal is then filtered by BPF 107 and amplified by another amplifier 108 prior to downconversion in a mixer 109. The frequency of the downconversion tone applied to the mixer 109 is controlled by a voltage controlled oscillator (VCO) 110. The output of the VCO 110 is filtered by a BPF 111 and amplified by an amplifier 112.
After downconversion, the downconverted signal is positioned at an intermediate frequency (IF) determined by the downconversion tone applied to the mixer 109, and additional gain is provided by an amplifier 113. A BPF 114 serves as an anti-aliasing filter prior to a second downconversion operation that, as recognized by the present inventors, may be performed with the sampling ADC 115. After the second downconversion operation, the signal is positioned near baseband (i.e. near 0 Hz).
The digital signal processor (DSP) 116 performs various operations including despreading the signal if it is a spread spectrum signal and then sends the baseband signal to a demodulation block 117 so as to extract the data originally added to the transmitted signal by the transmitter. Frequency control of the DSP 116 is provided by an oscillator 118 and using the DSP 116 passes the frequency control to all relevant sections 110, 115 and 117 to compensate for mismatch in received signal frequency, and/or chip phase, though frequency mismatch is usually minimized by using accurate frequency references at the transmitter and receiver. While FIG. 1 depicts a single RF downconversion step, other downconversion steps may be added to properly center the received signal at the desired ADC IF frequency.